1. Field of the Invention
This invention relates generally to a master cylinder for a brake and particularly to a hydraulic pressure control valve which is connected to an outlet port of the master cylinder and releases its hydraulic pressure control operation when a secondary piston of the master cylinder is moved excessively beyond a predetermined stroke.
2. Prior Art
FIG. 1 shows one conventional hydraulic pressure control valve of this type as disclosed in European Patent Application No. 0157158. The hydraulic pressure control valve comprises a body 16 mounted at one end in a bore 14 of a housing of a tandem master cylinder, the bore 14 communicating with a pressure chamber 12 defined by the housing and a piston 10. The body 16 has a bore 18 of a stepped shape in communication with the bore 14, and an outlet 20 disposed at the other end of the body 16 and communicating with the bore 18. A piston 24 of a stepped shape is slidably received in the bore 18 so as to receive a pressure in the outlet 20, the piston 24 having a central passage 22 extending therethrough. A preload spring 26 is mounted in the bore 18 so as to urge the stepped piston 24 toward the outlet 20. A valve member 30 is provided so as to seat on a valve seat 24a formed on one end of the stepped piston 24 closer to the bore 14, the central passage 22 opening to this one end of the stepped piston 24 The valve member 30 is normally held in a predetermined position away from the valve seat 24a by means of a valve spring 28. With this hydraulic pressure control valve, the stepped piston 24 is held by the preload spring 26 in the position shown in FIG. 1 until the hydraulic pressure in the pressure chamber 12 reaches a predetermined level, so that the valve member 30 is held away from the valve seat 24a. When the hydraulic pressure in the pressure chamber 12 exceeds the predetermined level, the stepped piston 24 is moved upwardly (FIG. 1) against the bias of the preload spring 26, due to the hydraulic pressure differential between the greater diameter portion 24b and smaller diameter portion 24c of the stepped piston 24. As a result, the valve seat 24a is brought into seating engagement with a lower portion 30a of the valve member 30. Therefore, the rate of increase of the hydraulic pressure leading to the outlet 20 and acting on a brake for rear wheels of the vehicle becomes smaller than that of the hydraulic pressure (which is the same as the hydraulic pressure for a brake for the front wheels) in the pressure chamber 12.
However, if such a pressure control is carried out even when a brake circuit for the front wheels is subjected to malfunction, braking forces are insufficient to brake the vehicle. For this reason, with the hydraulic pressure control valve shown in FIG. 1, when the piston 10 is moved excessively beyond a predetermined stroke, a flange 10a of the piston 10 is brought into engagement with an upper portion 30b of the valve member 30 to move the valve member 30 in a direction perpendicular to the axis of the valve member 30 so as to pivot the valve member about its lower end 30c, thereby preventing the valve member 30 from seating on the valve seat 24a.
With the conventional hydraulic pressure control valve of FIG. 1, however, in the event of the replacement of component parts or the like, the piston 10 can not be removed from the housing of the master cylinder without removal of the hydraulic pressure control valve from the master cylinder housing since the valve member 30 extends into the pressure chamber 12.